Non-destructive Testing for Composites: Different inspection methods

With more and more composite structures on aircraft, knowing how to inspect them for damage is an important topic. Not only are there more composite parts on aircraft, but in a case such as the Cirrus, essentially the entire aircraft structure is made from advanced composite materials. But just as in other aircraft material inspection, some types of inspections are more effective than others when dealing with composites. At a recent IA seminar, I sat in on a presentation by Greg Mellema of Abaris Training on composite inspection. This article is based on that presentation and will take a look at some different inspection techniques to consider when working with composites.

Visual inspection
A basic and useful part of inspection on composite structures is a visual inspection. The inspector looks for visible signs of damage to the structure like burns, disbonds, and delaminations. Of course, the inspector must have an understanding of composite materials in order to know what to look for.

Tap testing
This is another basic method of inspection. You tap a coin along the part's
surface. A bright, metallic sound indicates good structure. A dull "thud" would indicate a flaw such as a delamination or disbond. It is more effective on thinner structures, and loses its effectiveness on very thick laminates. Another disadvantage is that it won't go through core. It can also sometimes give false readings, say if another structure is attached on the back side.

Ultrasonic inspection
Ultrasonic inspection is another type of non- destructive testing that may be considered for inspection of composite structure. There are three basic types, A-Scan, C-Scan, and ANDSCAN.

A-Scan
A-Scan ultrasonic equipment uses a pulse-echo system to locate flaws.
A probe, or transducer, is placed on the surface to be inspected using a gel couplant to ensure good sound conductivity. The probe then transmits ultrasonic pulses through the part. It measures the time it takes to bounce from the back surface of the part. Flaws in the material like disbonds and water intrusion would result in a different time of flight. The user needs to know the material velocity (speed at which sound travels through a material) of the material being tested and input that into the tester.

C-Scan
C-Scan also uses ultrasonic sound to inspect parts. But unlike A-Scan, it measures the amount of time that the sound passes through the thickness of the part. For this reason, you need access to both sides of the part. In addition, it needs to be submerged in a liquid medium to ensure good sound transmission. This can cause water intrusion in some parts. But C-Scan does perform better than A-Scan on composites. It provides a good overall scan image of the part that the inspector can use to locate damage.

ANDSCAN
This product from Agfa NDT takes A-Scan to the next level. A mechanical
arm is attached to the transducer, which is connected to a computer. The arm provides the computer with x, y, and z coordinates in space for the transducer. This enables the computer to generate a C-Scan-like image of the inspection area. The computer also stores the information by tail number allowing for trend analysis.

X-ray inspection
X-ray inspections of composites are performed like those on metal structures. Images are based on material density. It can also see some delaminations if set up at an angle. Water and inclusions can also be seen.

Thermography
Thermography can be used to inspect composite structures. In its most
basic form, heat is applied to a part and then the part is viewed through an infrared camera as it cools. This can give indications of a flaw to a trained eye.
A new type of thermography being marketed by Thermal Wave Imaging is Pulsed Thermography. The equipment uses a pulse of high intensity light to quickly heat up the surface of the part. Then as the part cools, it is analyzed
by an infrared camera and proprietary software called MOSAIC. The equipment can piece together separate tests and put together a "mosaic" of the piece so that the whole part can be viewed.

Training
In the end, it all comes down to training. Many companies offer training on composite inspection and repair. In addition, training is available on the different types of testing equipment mentioned. The more knowledgeable you are on inspection of composite structures and how to use the various inspection tools, the more comfortable you can be that the aircraft or component you are inspecting is airworthy.

High-tech tap tester
WichiTech's RD3 electronic digital tap hammer has taken composite tap testing to the next level. It uses an accelerometer in the head of the
hammer that is hooked up to a computer to measure the response time of the hammer head. This is helpful in high noise environments since the user does not need to hear the sound, the hammer itself measures it and provides a numerical display.